This invention relates to a process for the injection, by pneumatic means, of metered amounts of pulverulent materials at different points into a vessel which is under variable pressure. As a result of this process, a pneumatic mixture is formed which has a relatively high content of pulverulent materials in the propulsion air. Also, a supercritical speed is imparted to the pneumatic mixture before it is injected into the vessel. This invention also relates to an apparatus for utilizing the above-described process.
German patent document DE-A-No. 33 25 901, corresponding to U. S. Pat. No. 4,570,552, assigned to the assignee hereof, the entire contents of which is incorporated herein by reference, proposed a process of the type hereinabove described wherein a pneumatic current is accelerated to supercritical speed, that is, the speed of sound, which varies in dependence on the density of the current. The acceleration of the pneumatic current to a supercritical speed has the advantage that the conditions of the flow upstream of the point where the supercritical speed is obtained are not influenced by the conditions of flow downstream of that point. This provides an important advantage in that it is no longer necessary to take into account the fluctuations of pressure and other variable parameters existing in the vessel into which the pulverulent materials are injected.
Although not limited to this application, the present invention will be described more particularly with reference to its preferred application, that is, the injection of solid fuels, such as powdered coal, into an industrial furnace, such as a blast furnace.
It is known that the base of the blast furnaces include a circular array of tuyeres, each of which receives hot air under pressure through blast pipes connecting the tuyeres to a circular hot air supply pipe. The solid fuels required for maintaining the reduction process in the furnace are generally injected directly into the tuyeres or the blast pipes.
The abovementioned U.S. Pat. No. 4,570,552 proposes to dispose the point where the pneumatic mixture is accelerated to supercritical speed at the location where injection of the pneumatic mixture is introduced into each blast pipe. Consequently, it is necessary to provide as many metering means as there are blast pipes, as well as individual pipes for carrying the mixture between each metering means and each blast pipe. However, taking into account the circular configuration of the blast furnace and its array of blast pipes, all the pipes carrying the mixture between the metering means and the blast pipes, or at least all the pairs of pipes, have different lengths. These different lengths obviously entail pressure drop differences which modify the conveying conditions from one pipe to another.